GB2065104A

GB2065104A - Liquid Crystal Bicyclo-octane Compounds

Info

Publication number: GB2065104A
Application number: GB8034263A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1979-10-26
Filing date: 1980-10-23
Publication date: 1981-06-24
Also published as: GB2065104B

Abstract

A novel liquid crystal compound has the molecular structure <IMAGE> where R<1> is an alkyl group, R<2> is an alkyl group an alkoxy group, an alkanoyloxy group or an alkoxycarbonyloxy group <IMAGE> is a 1,4 disubstituted bicyclo-(2,2,2)- octane ring, and <IMAGE> is a 1,4 disubstituted benzene ring, n being an integer, preferably 1 or 2.

Description

SPECIFICATION Liquid Crystal Bicyclo-octane Compounds and Materials and Devices Containing Them The present invention relates to liquid crystal bicyclo-octane compounds and materials and devices containing them.

In the field of displays there is a requirement for electro-optic devices having a low power consumption. Devices incorporating liquid crystal materials have been shown to satisfy this requirement because they have an electrically switchable molecular arrangement and a very large electrical resistance, and at the present time a considerable amount of interest is being shown in such devices for these reasons.

There are many known liquid crystal materials; some have been known for many years. Liquid crystal materials are organic materials which exhibit a liquid crystal phase in which the molecules are arranged over limited spatial ranges in an ordered structure.

The materials may be single compounds or mixtures of compounds.

According to the present invention there is provided a novel bicyclo-octane liquid crystal compound having the following molecular structure:

where R1 is an alkyl group, R2 is an alkyl, alkoxy, alkanoyloxy or alkoxycarbonyloxy group,

is a 1,4 disubstituted bicyclo(2,2,2)octane ring and

is a phenyl ring, n being an integer greater than 0.

A compound (or compounds) having molecular structure (I) will be referred to as a compound (or compounds), as defined.

The groups R' and R2 preferably contain less than eighteen carbon atoms, eg one to ten carbon atoms, and may be normal or branched. If R1 and/or R2 are branched they may contain a chiral centre in which case the compound is optically active.

Preferably n is 1 or 2 so that the compound is either

By a 'liquid crystal compound' is meant a compound in one of the following two known categories: (i) Compounds which normally exhibit a liquid crystal phase; (ii) Compounds which do not normally exhibit a liquid crystal phase but which nevertheless usefuliy affect some aspect of liquid crystal behaviour when dissolved in other liquid crystal compounds.

Compounds in category (ii) show a 'monotropic' or 'virtual' liquid crystal to isotropic liquid transition at a temperature below the melting point of their solid phase. The monotropic or virtual transition may be detected respectively by rapid cooling of the liquid phase or by dissolving the compound in a material exhibiting a liquid crystal phase, observing the change in the transition to the isotropic liquid phase of the material by the addition and calculating the virtual transition temperature by extrapolation.

Compounds in category (ii) might for example be usefully dissolved in other liquid crystal compounds to extend or vary the liquid crystal temperature ranges of the compounds or to vary the molecular helical pitch (in the case of 'cholesteric' liquid crystals).

Material which exhibits a liquid crystal phase and either consists of or contains a compound as defined above will be referred to herein as a material 'as defined'.

The liquid crystal phase exhibited by the material as defined may be nematic, smectic or cholesteric.

For a nematic phase the compound as defined should have a normal alkyl group R whilst for a cholesteric phase the compound as defined should have a branched alkyl group R containing a chiral centre.

Preferably compounds as defined are prepared from the intermediate bromo compound

eg by one of the following procedures:

A compound as defined may be mixed with one or more other compounds as defined to enhance its liquid crystal properties, eg to extend its liquid crystal temperature range or otherwise to modify the properties, eg the electrical properties of the liquid crystal phase in an advantageous manner.

For example the following are examples of such mixtures:

RA2 etc and R' etc contain 410 carbon atoms and are various examples of R2 and R1 as defined above.

Likewise, compounds as defined may be mixed with one or more of the compounds in the following categories:

where

is a cyclohexane ring,

is a bicyclo(2,2,2)octane ring, X is a 1,4 phenylene group

or a 4,4' biphenylyl group

or a 2,6 naphthyl group

and Y is CN, or R4, or OR4 or CO.O--XX-Y1 where Y1 is CN, or 1R5 or OR5; the definition of R,R4,R5 being the same as that of R'.

According to another aspect of the present invention a liquid crystal device includes means for containing a region of liquid crystal material, a region of liquid crystal material contained in the containing means, wherein the liquid crystal material is material as defined above, and means for applying an external stimulus to the material to alter the molecular arrangement in the material.

The means for applying an external stimulus may for example be means for applying an electric field to the region.

The change in molecular arrangement produced by the external stimulus may be used to affect the transmission of electromagnetic radiation at any wavelength of interest through the material, for example, by changing the transmissivity of the material or by changing the helical order of the material, if it is optically active.

According to another aspect of the invention, an electro-optic liquid crystal device includes two adjacent plates at least one of which is optically transparent, a layer of liquid crystal material contained in the space between the plates, wherein the liquid crystal material is the material as defined above, and deposited on the inner facing surfaces of the plates, a film of conducting material to allow an electric field to be applied across the layer. The electro-optic device may for example be a display device used, for instance, in an instrument such as a time watch or clock. The device may operate by one of the following modes: (a) twisted nematic effect (b) (cholesteric-to-nematic) phase change effect, Freedericksz effect, two frequency switching effect, dynamic scattering effect or cholesteric memory effect.

Compounds as defined above are, in general, chemically and photochemically stable liquid crystal compounds which have a low dielectric anisotropy and low birefringence.

The low dielectric anisotropy makes them particularly attractive for use in multiplexed twisted nematic devices together with materials of positive dielectric anisotropy, eg mixtures of: (1) 4'-n-alkyl- or alkoxy-cyanobiphenyls (preferably containing less than 5 carbon atoms in the alkyl or alkoxy group) and/or one or more 1 -(4-cyanophenyl)-4-alkyl-bicyclo (2.2.2)octanes together with (2) One or more of the following:

Preferably the compound(s) from group (1) comprise 1090% of the mixture by weight, typically 60% by weight, the compounds from group (2) comprise 0-1 5% by weight, typically 10%, the remainder (excluding any further additional compounds) being compound(s) as defined.

The compounds as defined are more attractive for this purpose than, for example, the benzoate esters used in the invention of UK Patent Application No 7931563 because the compounds as defined are generally more stable.

The low birefringence makes the compounds as defined particularly attractive for use in cholesteric-to-nematic phase change effect devices, eg typically 10% by weight in mixtures with known materials, to reduce the birefringence of such materials.

The low dielectric anisotropy makes the compounds as defined useful for blending with other materials to form mixtures having a reduced (positive or negative) dielectric anisotropy. Examples of materials and devices in which the compounds as defined may be used in this way are any of those given above. It will be apparent to those skilled in the art that the dielectric anisotropy of the mixture can be controlled by appropriate control of the relative proportions of the materials blended together.

Examples of methods of preparing compounds as defined will now be described. In these examples 'MP' represents melting point, 'C-N' represents crystalline to nematic liquid crystal transition temperatures and 'N-I' represents nematic liquid crystal to isotropic liquid transition temperature.

'C-I' represents crystalline to isotropic liquid transition temperature and 'S6-N' represents smectic B to nematic transition temperature.

Example 1 The intermediate compound 1-alkyl substituted 4-bromobicyclo [2,2,2] octane which is used in the preparations described below may be prepared by the following route:

Step Al: The production of 3-acetyl-1 ,5-dicyano-3 substituted pentane.

Step B1: The production of 3-acetyl-3-substituted pentane-1 ,5-dicarboxylic acid.

Step Cl: The production of 4-acetyl-4-substituted cyclohexanone.

Step D1: The production of 4-hydroxy-1-substituted bicyclo 2.2.2 octan-2-one.

Step El: The production of 1-hydroxy-4-substituted bicyclo 2.2.2 octane.

Step F1: The production of 1-bromo-4-substituted-bicyclo 2.2.2 octane.

All six of these steps may be carried out by methods essentially analogous to those for R1=methyl and ethyl described by H D Holtz and L M Stock in the Preparation of 1 -Carboxy-4-Substituted Bicyclo 2.2.2 Octanes, J. Am. Chem. Soc, 86, 5183 (1964).

Example 2 The production of 1-(4'-alkyl- or -alkoxy-substituted phenyl)-4-alkyl substituted bicyclo(2,2,2) octanes by the following procedure.

where R' and R2 are as previously defined Step A2 A solution of 1-bromo-4-substituted bicyclo(2,2,2)octane (0.0058 mol) in sieve-dried nitrobenzene (35 cm3) is added dropwise over a period of thirty minutes to a well-stirred solution of sieve-dried monosubstituted benzene (C8H5R2) (0.2326 mol) and crushed, anhydrous aluminium chloride (0.0023 mol) in sieve-dried nitrobenzene (35 cm3) maintained at 800C under anhydrous conditions. The mixture is then left stirring at this temperature, eg, overnight. It is then added to a small volume of an hydrochloric acid/ice/water mixture and stirred for about twenty minutes. The organic layer is separated off and steam distilled to remove the solvent and monosubstituted benzene.The residue is dissolved in chloroform and the resultant solution is dried over anhydrous magnesium sulphate. The filtered solution is then evaporated down to dryness under vacuum. The solid residue is purified by column chromatography on silica gel using a 1.5:1 mixture of choroform and light petroleum (bp 40-600C) as eluent.

Fractions giving a single spot by tic are collected and evaporated down together under vacuum to yield a solid which is crystallised from methanol.

Examples of such a product are:

C--I=42 OC; N-l=(7 0C)* C-I=49.5 0C; N-I=(3 1 C)*

C-N=640C; N--I=700C C--I=69 OC; N-I=(470C)* C=solid N=nematic S=smectic B I=isotropic liquid C-I etc=transition temperature )* represents a monotropic 'virtual' transition temperature obtained by studying suitable binary mixtures of the compound with a standard material.

Example 3 The production of 1-(4'-alkyi- or -alkoxy-substituted 4"-biphenylyl)-4-alkyl substituted bicyclo(2,2,2)octane by the following procedure.

where R1 and R2 are as previously defined Step A3 A solution of 1-bromo-4-substituted bicyclo(2,2,2)octane (0.004 mol) in sieve-dried nitrobenzene (50 cm3) is added dropwise over ninety minutes to a well-stirred solution of 4-substituted biphenyl (0.012 mol) and crushed, an hydros aluminium chloride (0.0016 mol) in sieve-dried nitrobenzene (50 cm3) maintained at 800C under anhydrous conditions. The mixture is then left stirring at this temperature, eg overnight.

The reaction mixture is worked up as described above in Example 2 and the product is crystallised from 2-methoxyethanol.

Examples of such products of the form

are as follows: 1 ) R,=C2H5 R2=n-C3H7 C--N=1 250C; SBN=(1240C); N--l=l 550C 2) R,=CH3 R2=n-C7H15 C--l=106 C; SB=(105 C) 3) R1=n-C4H9; R2=n-C3H7 C--SB=160 C:SB--N=1 79.50C; N--I=208 C 4) R=nC6Ht; R2=n-C3K7 S1-N=1 91 OC; N-l=2 1 00C 5) R1=n--C5H11; R2=n-C5H11 6) R1=n--C3H7; R2=n-C3H7 7) R1=n-C3H7; R2=n-C5H11 represents a monotropic transition temperature obtained on a cooling cycle.

Example 4 The production of compounds wherein R2=alkanoyloxy and alkoxycarbonyloxy

R3=alkyl Step A4 The appropriate methyl ethers (n=1 or 2) are demethylated by standard demethylation procedures to yield the appropriate phenols (n=1 or 2).

Step B4 The phenols produced in Step A4 (n=1 or 2) are converted into the corresponding esters (n=1 or 2) by a standard esterification method involving the use of the acid chloride R3COCI.

Step C4 The phenols (n=1 or 2) produced in Step A4 are converted into the corresponding carbonate esters (n=1 or 2) by standard methods involving the use of the alkyl chloroformate (CICOOR3).

These results illustrate that the compounds according to formula (I) have low or negative dielectric anisotropes.

Examples of devices embodying the invention will now be described by way of example only with reference to the accompanying drawings wherein: Figure 1 is a sectional view of a twisted nematic digital display; Figure 2 is a sectional view of the display shown in Figure 1; Figure 3 shows a rear electrode configuration for Figure 1; Figure 4 shows a front electrode configuration for Figure 1; Figures 5, 6, 7 show schematic views of the device of Figures 1 to 4 with typical addressing voltages.

The display of Figures 1 to 4 comprises a cell 1, formed of two, front and back, glass slides 2, 3 respectively, spaced about 7 ,um apart by a spacer 4 all held together by an epoxy resin glue. A liquid crystal material 1 2 fills the gap between the slides 2, 3 and the spacer 4. In front of the front glass slide 2 is a front polariser 5 arranged with its axis of polarisation axis horizontal. A reflector 7 is arranged behind the slide 3. A rear polariser 6 or analyser is arranged between the slide 3 and reflector 7.

Electrodes 8, 9 of tin oxide typically 1 00 A thick are deposited on the inner faces of the slides 2, 3 as a complete layer and etched to the shapes shown in Figures 3, 4. The display has seven bars per digit 10 plus a decimal point 11 between each digit. As shown in Figure 3 the rear electrode structure is formed into three electrodes x,, x2, x3. Similarly the front electrode structure is formed into three electrodes per digit and decimal point y,, y2, y3 . ... . Examination of the six electrodes per digit shows that each of the eight elements can independently have a voltage applied thereto by application of suitable voltage to appropriate x, y electrodes.

Prior to assembly the slides 2, 3 bearing the electrodes are cleaned then dipped in a solution of 0.2% by weight of poly-vinyl alcohol (PVA) in water. When dry, the slides are rubbed in a single direction with a soft tissue then assembled with the rubbing directions orthogonal to one another and parallel to the optical axis of the respective adjacent polarisers, ie so that the polarisers are crossed.

When the nematic liquid crystal material 12 is introduced between the slides 2,3 the molecules at the slide surfaces lie along the respective rubbing directions with a progressive twist between the slides.

When zero voltage is applied to the cell 1 light passes through the front polariser 5, through the cell 1 (whilst having its plane of polarisation rotated 900) through its rear polariser 6 to the reflector 7 where it is reflected back again to an observer, (shown in Figure 1 at an angle of 450 to the axis Z normal to axes X and Y in the plane of the slides 2, 3). When a voltage above a threshold value is applied between two electrodes 8, 9 the liquid crystal layer 12 loses its optical activity, the molecules being re-arranged to lie perpendicular to the slides 2, 3, ie along the axis Z. Thus light at that position does not reach the reflector 7 and does not reflect back to the observer who sees a dark display of one or more bars of a digit 10.

Voltages are applied as follows as shown in Figures 5, 6 and 7 for three successive time intervals in a linescan fashion. An electric potential of 3V/2 is applied to, ie scanned down, each x electrode in turn whilst -V/2 is applied to the remaining x electrodes. Meanwhile3V/2 or V/2 is applied to the y electrodes. A coincidence of 3V/2 and -3V/2 at an intersection results in a voltage 3 V across the liquid crystal layer 12. Elsewhere the voltage is V or-V. Thus by applying3V/2 to appropriate y electrodes as 3V/2 is scanned down the x electrodes selected intersections are turned ON as indicated by solid circles. The electric voltage V is an ac signal of eg 100 Hz square wave, and the sign indicates the phase.

It will be apparent to those skilled in the art that the device shown in Figures 1 to 7 is a multiplex display because the electrodes are shared between ON and OFF intersections or display elements.

Each OFF intersection receives V for the entire scan period whilst each ON intersection receives 3 V for one third of the scan period and V for the rest of the scan period making a rms value of 1.91 V.

The material of the layer 1 2 needs to be such that the voltage required to achieve the ON state is less than 1.91 times the voltage giving the OFF state to provide a suitable contrast between the ON and OFF states. (The ratio of these two voltages for a given material may be designated as the parameter M20 which is defined below.) This illustrates that the properties of the material are important in determining the multiplexability of the device.

For similar displays having more than three rows of electrodes the figure of 1.91 V is reduced accordingly.

A material embodying the present invention which is suitable for use as the material 1 2 in the above device is as follows:

c2H55cN 37.5% Mixture B S C4H9#m EN 37.5% 60% by weight C3H7OmThOCN 25% Compound C

15% by weight Compound D

15% by weight

9% by weight Compound E

(Rc=2-methylbutyl) 1% by weight (known) Addition of

to the compounds C, D and E and the biphenyl Mixture B reduces the temperature dependence ot the threshold voltage of the device described above.This effect, and the improvement in multiplexibility of a matrix-type liquid crystal display of the twisted nematic kind, using a mixture of this form, is further described in a copending UK Patent Application.

In an alternative embodiment of the invention a (cholesteric-to-nematic) phase change effect device incorporates a material as defined above.

A cell is prepared containing a long helical pitch cholesteric material sandwiched between electrode-bearing glass slides as in the twisted nematic cell described above. However, the polarisers and surface preparations for homogeneous alignment, eg treatment of the glass slide surfaces with SiO, are not used in this case.

If the glass slides are untreated and the liquid crystal material has a positive dielectric anisotropy (hE) the liquid crystal material is in a twisted focal conic molecular texture in the OFF state which scatters light The effect of an electric field applied between a pair of electrodes on the respective inner surface of the glass slides is to convert the region of liquid crystal material between the electrodes into the ON state which is a homeotropic nematic texture which is less scattering than the OFF state.

If the inner glass slide surfaces are treated, eg with a coating of lecithin, to give alignment perpendicular to those surfaces, and the liquid crystal material has AE negative the material in the OFF state is in a homeotropic texture which has little scattering effect on incident light. If an electric field is applied between a pair of electrodes on the respective inner surfaces of the glass slides the region of liquid crystal material between the electrodes is converted to a twisted homogeneous texture which scatters light (the ON state).

The contrast between the two states in each case may be enhanced by the addition of a small amount of a suitable pleochroic dye (eg 1% by weight of 1 ,5-bis-4'n-butylphenylamino anthraquinone in the case where hE is positive) to the liquid crystal material.

A suitable positive dielectric anisotropy material embodying the invention for use in a phase change effect device is:

E2H5ffiThOOCN 37.5% Mixture B < } C4Hg H con 37.5% 60% by weight l C3H70 < CN 25%

(Rc=2-methylbutyl) 10% by weight Compound C (see above) 10% by weight Compound D (see above) 10% by weight

30% by weight A suitable negative dielectric anisotropy material embodying the invention is:

10% by weight Compound C (see above) 35% by weight Compound D (see above) 35% by weight

9% by weight 10% by weight 1% by weight (Rc=2-methylbutyl)

Claims

1. A bicyclo-octane liquid crystal compound having the following molecular structure:

is a 1,4 disubstituted bicyclo(2,2,2)octane ring and

is a phenyl ring, n being an integer greater than 0.

2. A compound as claimed in claim 1 and wherein the integer n is 1 or 2.

3. A compound as claimed in claim 2 and wherein the alkyl groups contained in or constituting the groups R1 and R2 have 18 or less carbon atoms.

4. A compound as claimed in claim 3 and wherein the alkyl groups contained in or constituting the groups R1 and R2 are n-alkyl groups.

5. A compound as claimed in claim 3 and wherein at least one of the alkyl groups contained in or constituting the groups R1 and R2 is a branched alkyl group.

6. A compound as claimed in claim 5 and wherein the branched alkyl group has a chiral centre.

7. A compound as claimed in any one of the preceding claims and wherein the integer n is 1, the compound having a molecular structure:

8. A compound as claimed in any of the preceding claims and wherein the integer n is 2, the compound having a molecular structure:

9. A compound as claimed in claim 1 and having a formula:

10. A compound as claimed in claim 1 and having a formula:

11. A compound as claimed in claim 1 and having a formula:

12. A compound as claimed in claim 1 and having a formula:

13. A compound as claimed in claim 1 and having a formula:

14. A compound as claimed in claim 1 and having a formula:

15. A compound as claimed in claim 1 and having a formula:

16. A compound as claimed in claim 1 and having a formula:

17. A compound as claimed in claim 1 and having a formula:

18. A compound as claimed in claim 1 and having a formula:

19. A compound as claimed in claim 1 and prepared by a method substantially the same as in Example 2,3 or 4 hereinbefore described.

20. A liquid crystal electro-optic device including two adjacent plates at least one of which is optically transparent, a film of liquid crystal material contained between the plates and deposited on the inner surface of the plates electrodes to allow electric fields to be applied across the film, characterised in that the liquid crystal material includes one or more compounds as claimed in claim 1.

21. A device as claimed in claim 20 and which is one of the types of device specifically described hereinbefore.